1. Technical Field
The present disclosure relates to electronic circuits and, in particular, to a filtering circuit with coupled acoustic resonators.
2. Description of the Related Art
Today acoustic resonators are widespread in consumer applications as well as in professional applications because of their high efficiency, including the realization of high performance band pass filters used in mobile telephony.
Research has been made on two types of acoustic resonators that show remarkable performance, namely the SAW resonators (Surface Acoustic Wave Resonator) and, more recently, those of type BAW (Bulk Acoustic Wave Resonator). In the first type, the acoustic resonator is located on the surface of a semiconductor product while, in BAWs, it lays inside a volume delimited between a lower electrode and a higher electrode so that the acoustic wave develops in this volume. BAW resonators are the subject of substantial research by manufacturers of semi-conductor products because those components allow for a high degree of integration of filtering circuits, thus consequently anticipating significant manufacturing cost savings.
BAW resonators allow higher frequencies than those used with SAWs, while achieving more compact structures.
Conventionally, BAW resonators are combined so as to form more complex structures, such as “ladder” or “lattice” in order to achieve highly effective band pass filters.
Another known combination of resonators is based on the coupling of several resonators in order to achieve a more complex structure, such as the Coupled Resonator Filter (CRF), which is illustrated in FIG. 1.
This circuit includes, as illustrated in the figure, two structures or stages perfectly symmetrical with respect to an axis of symmetry running along the middle of the figure.
A first stage comprises an upper resonator having two electrodes, respectively a bottom electrode 11 and a top electrode 12 separated by a layer of piezoelectric material 7. The structure is located above a layer 6 performing an acoustic coupling, which is located on a lower resonator with two electrodes, respectively a bottom electrode 3 and an top electrode 5 separated by a layer of piezoelectric material 4.
On the other side of the vertical axis, and in perfect symmetry with the first stage, the circuit includes a second stage comprising an upper resonator and a lower resonator separated by the layer of an acoustic coupling 6. The upper resonator includes two electrodes, respectively an bottom electrode 21 and a top electrode 22 separated by layer 7. The lower resonator includes electrodes 3 and 5 encompassing the layer 4.
The two-stage structure is arranged on an acoustic mirror 2, which is itself located on a silicon substrate 1 or SiGe, possibly comprising logic and analog MOS or CMOS circuits.
This so-called CFR structure is well known to those skilled in the art and will not be further described as to its structure or its manufacturing process. Briefly, the upper resonator (electrodes 11 and 12 and layer 7) receives the electrical signal to be filtered and such signal is converted into an acoustic wave which is a volume wave.
This acoustic wave propagates from top to bottom via acoustic coupling layer 6, to the lower resonator of the first stage where it is converted into an electric signal which is then forwarded to the lower resonator of the second stage since the latter shares the same electrodes than the lower resonator of the first stage.
The wave volume then propagates up to the layers of the second stage and, through the acoustic coupling of layer 6, reaches the upper resonator of the second stage, which is located to the right of FIG. 1.
FIG. 2 shows the actual path of the wave volume within the two structures, left and right respectively, of circuit CRF, and through successive coupling, electro-mechanical and also mechanical.
FIG. 3 particularly illustrates a coupling chain carried out in the CRF filter. The upper and lower resonators are represented respectively by items 32 and 34 of FIG. 3. One may see that the electrical signal that is input in the first stage is subject to an electro-mechanical conversion (Kem), thus resulting in a transfer of electrical energy to mechanical energy represented by a block of electro-mechanical coupling Kem 31.
The volume wave is transmitted to the lower resonator of the first stage, through the layer 6, which provides a purely mechanical coupling Km, represented by the block 33.
The lower resonator of the first stage receives this wave and converts the received mechanical energy into an electrical energy resulting in an electrical signal to terminals 3 and 5 of right lower resonators.
This electrical signal is then transmitted to the lower resonator of the second stage because it shares the same electrodes as the lower resonator of the first stage.
FIG. 4 illustrates more particularly the comparison of the filtering curve of a CRF circuit with a classical group of resonator BAW of type <<scale>>.
At equal bandwidth, the CRF circuit provides a rejection rate higher than that of a classical SCALE, and especially far from the bandwidth. This produces an efficient filtering over a wideband frequency.
This performance gain of CRF circuit is obtained together with a limitation of the space required in the semiconductor circuit since the CRF structure allows for stacking two BAW resonators to form a single stage.
This circuit can also be easily combined with another CRF circuit to make a filtering circuit with two impedances, respectively of input Zin and of output Zout, separate.
To this end, it combines two CRF circuits by connecting the input floors in parallel and the output floors in series.
Finally, the filter with coupled resonators ease the conversion to a differential structure.
Despite these advantages, the CRF circuit shows, furthermore a difficulty in obtaining a higher achievement, which is a serious disadvantage.
Indeed one may observe that in a neighborhood near the bandwidth, the circuit selectivity shows a slope that is less steep than the one resulting from the LADDER structure. Such a drawback is critical in the field of wireless communications based on the use of different—but close—frequency bands that need to be efficiently filtered.
Particularly, in the case of mobile communications of the new generation, it is planned to arrange two bands of frequency that are very closed, a first band for broadcast communications and a second band for the reception and, clearly, in such a context, the CRF shows to be unsuitable.
Obviously, it has been considered to combine different structures of identical CRF filters in order increase the overall order of the band-pass filter and, therefore, to improve the selectivity of the filtering process in the vicinity of the band. However, this solution would lead to a significant increase in surface area on the semiconductor substrate, and more importantly, it would lead to increased losses in the filter reducing the coefficient quality of the filter.
Such is the problem which is addressed by the present disclosure.